In recent developments of telecommunications technology (and in the development of integrated services broadband networks (B-ISDN) in this case), the asynchronous transfer mode (ATM) based on asynchronous time-division multiplex plays a significant part. The signal transmission therein proceeds in a bit stream that is subdivided into cells (each respectively composed of a header and a useful information part) having a constant length of, for example, 52 octets that are occupied as needed with packeted messages. When no useful information is to be communicated at the moment, then specific dummy cells are transmitted. Virtual connections, i.e. connections that only in fact use a path section when a message packet (block) is in fact to be communicated thereover, are set up in ATM switching centers, whereby the header of every packet contains, among other things, an address covering, for example, 2 octets for unambiguous allocation of the packet to a specific virtual connection. Dependent on the technique of the respective selection information, every packet at the input to the switching network can obtain the complete information for its path through the switching network. With the assistance of this information, the switching elements then connect through the packet on the defined route themselves (self-routing network) (see, for example, Telcom Report 11 (1988)6, 210. . . 213).
An ATM network offers the subscriber an extremely high maximum bit rate r.sub.max of, for example, 150 Mbit/s, whereby only a part of the available bit rate is often utilized by the subscriber dependent on the respective ATM service, namely
in the form of the constant bit rate r.sub.c (of, for example, 2 Mbit/s) in what are referred to as CBR (constant bit rate) connections; PA1 in the form of the average bit rate r.sub.a (of, for example, 5 Mbit/s) and of the peak bit rate r.sub.p (of, for example, 30 Mbit/s) in what are referred to as VBR (variable bit rate) connections.
During the call set up, the respectively desired traffic parameters are negotiated and potentially declared. A decision is thereby made in a call acceptance control means as to whether the momentarily available network capacity is adequate in order to set up a desired, virtual connection.
After the set up of the virtual connection, the negotiated traffic parameters are monitored in a bit rate monitoring means (policing unit). When the negotiated value is exceeded, then the excess ATM cells are eliminated. However, it is also possible to mark the excess ATM cells such that they can still be subsequently eliminated in the network as low-priority ATM cells in case traffic jams arise.
The bit rate monitoring usually occurs according to what is referred to as the leaky bucket principle (see, for example, Niestegge, the "Leaky Bucket" Policing Method in the ATM (Asynchronous Transfer Mode) Network, INTERNATIONAL JOURNAL OF DIGITAL AND ANALOG COMMUNICATION SYSTEMS, Vol. 3 (1990), 187 . . . 197). At every reception of an ATM cell, the reading of a counter is incremented by one place and (dependent on the negotiated traffic parameters), the counter reading is decremented by d places at specific intervals T. The counter reading 0 is thus neither downwardly crossed nor is a counter reading threshold s upwardly crossed. When the threshold is reached, the excess ATM cells are eliminated or marked.
When the subscriber adheres to the negotiated bit rate 4=B.sub.z .multidot.d/T, wherein B.sub.z is the plurality of "useful signal" bit per ATM cell, d is the decrementation value and T is the decrementation period, then the counter is always brought back to the counter reading of 0 due to the decrementation. When the subscriber exceeds the declared bit rate r, then the response threshold s is very quickly reached despite the decrementation.
Until the response threshold s is reached, the leaky bucket method allows a brief upward crossing of the declared bit rate. Proceeding from the counter reading of 0, thus s ATM cells are first allowed to pass by the leaky bucket unit in an uninterrupted sequence (i.e. without dummy cells or cells belonging to other connections) until the response threshold s of the counter is reached. When the counter reading is decremented by a value d immediately before this is reached, then further d ATM cells can pass. When the counter reading has been decremented a plurality of times before the response threshold s is reached, the plurality of ATM cells allowed to pass increases correspondingly.
The maximum possible plurality of ATM cells in what is referred to as a full-rate burst wherein the maximum bit rate of, for example, 150 Mbit/s occurs, is then n.sub.max =s +k.multidot.d with k=1+(s-d)/((T/t.sub.z)-d), wherein t.sub.z denotes the duration of an ATM cell at the maximum transmission rate and . . . denotes that only the whole-numbered part of the expression with . . . , is relevant.
One job of the bit rate monitoring is to limit the length of such full-rate bursts because full-rate bursts increase the probability of a buffer overflow in the ATM network. For this reason, the response threshold s should be as low as possible.
Given too low a response threshold, on the other hand, it could occur that subscribers that behave properly with respect to the reported bit rate nonetheless have ATM cells eliminated without justification, namely due to waiting time jitter, i.e. variable delays arising in queues.
For example, let a variable delay .delta. of, for example, 0.2, ms be permitted in the line circuit area, whereby the crossing probability is allowed to be 10.sup.-10. It follows therefrom that the leaky bucket counter dare not yet have reached the response threshold s after the appearance of this delay. The counter reading of n=s-1 that is just still permissible derives from the ATM cells received in the time span .delta.+T in the leaky bucket unit, the plurality of these ATM cells being d=.delta..multidot.r/B.sub.z. It is thereby then assumed that the counter reading was decremented to n=0 during the delay time span .delta. and that (in the worst case) a decrementation event occurs immediately before the arrival of the delayed ATM cell bundle. s=1+d+.delta..multidot.r/B.sub.z is then valid (also see Niestegge, op. cit. equation (6)). When s is defined in this way, then the probability of an unjustified elimination of ATM cells remains below the value 10.sup.-10. Examples of parameter values r, T, .delta., s, n.sub.max may be found, for example, in Niestegge, op. cit., Tables I and II.
Proceeding beyond the simple leaky bucket method outlined above, policing methods are currently preferred that make use of what is referred to as a dual leaky bucket principle. For that purpose, a chain of two leaky bucket units is provided, whereby the peak bit rate r.sub.p =B.sub.z .multidot.d.sub.1 /T.sub.1 is monitored with the first leaky bucket unit and the maximum possible plurality n.sub.max =s.sub.1 +k.multidot.d.sub.1 of ATM cells to the response of the first leaky bucket unit, in what is referred to as a full-rate burst, is defined. Here, B.sub.z =plurality of bits per ATM cell, d.sub.1 =decrementation value, T.sub.1 =decrementation period of the first leaky bucket unit, k=1+(s.sub.1 -d.sub.1)/(T.sub.1 /t.sub.z), s.sub.1 =response threshold of the first leaky bucket unit, and t.sub.z =duration of an ATM cell at the maximum transmission rate. The average bit rate r.sub.a =B.sub.z .multidot.d.sub.2 /T.sub.2 is monitored with the second leaky bucket unit, and the maximum duration t.sub.max .apprxeq.S.sub.2 .multidot.B.sub.z /(r.sub.p -r.sub.a) of a burst with the peak bit rate r.sub.p (peak rate burst) is defined (Niestegge, op. cit., Chapter 4.2). Here, d.sub.2 =decrementation value, T.sub.2 =decrementation period of the second leaky bucket unit, and s.sub.2 =response threshold of the second leaky bucket unit.
For these jobs, the second leaky bucket unit must react inertly, or must work within an extremely high threshold S.sub.2 ; when a service is defined, for example, with r.sub.a =2 Mbit/s, r.sub.p =10 Mbit/s and t.sub.max =2 s, then (given B.sub.z =48.multidot.8=384) a value of approximately 40,000 derives for s.sub.2. An unallowable increase in the average bit rater r.sub.a to, for example, 2.5 or 3 or 4 or 6 Mbit/s is thereby only recognized extremely late, after 32 or 16 or 8 or 4 s in the example, whereby approximately 40,000 ATM cells too many are transmitted.
What is disadvantageous, on the other hand, is a relatively frequent, unjustified elimination or marking of ATM cells at the t.sub.max monitoring. When the bit rate before a bit rate peak remains at the declared value r.sub.a for the average bit rate for a longer time, then the counter of the second leaky bucket unit is held roughly at the counter reading 0 due to the decrementation. During a bit rate peak of, for example, r.sub.p =5.multidot.r.sub.a that lasts for approximately the time span t.sub.max, the response threshold s.sub.2 be not quite reached yet. When no ATM cells arrive thereafter for some time, the counter (in a time span of approximately 5.multidot.t.sub.max) will be decremented to the counter reading 0.
When, however, another (allowed) bit rate peak occurs during this time (on average, at 2.5t.sub.max), then the counter starts again (on average at a counter reading s.sub.2 /2) and then very quickly reaches the response threshold s.sub.2 (on average, already after a time span of t.sub.max /2), with the consequence that s.sub.2 /2 (i.e., a few thousand) ATM cells are unjustifiably eliminated or marked. The situation becomes even worse when more than 2 bit rate peaks follow one another in a short time.